Why does my hen sometimes lay eggs with ridges halfway down the shell?
You may be describing what is called a "body checked egg." Body checks are ridges or grooves that occur around an egg, usually at the pointed end. They occur more often in older layers, and are not really a cause for concern to the backyard farmer. Commercially, they are considered a aesthetic flaw, but don't have any effect on edibility. Occasionally, the egg will even appear wrinkled all over or even misshapen due to a large number of checks.
Body checks are usually result of the hen's body attempting to repair any eggshell damage caused by stress when the egg was in the shell gland. This normally happens when there are disturbances in the coop. My girls all laid misshapen eggs once after a particularly bad thunder storm, and they were off laying for days after a helicopter flew too low over our house. Nightime predators may cause misshapen or checked eggs, too.
If for some reason you feel your chickens are ill--long periods of seriously misshapen eggs can indicate illness--you should take your chicken to a vet for a diagnosis.